1. Field of the Invention
The present invention relates to a threshold voltage generating circuit for generating threshold voltages, which are employed for discriminating signals in a semiconductor integrated circuit.
2. Description of the Background Art
FIG. 1 is a circuit diagram showing a conventional threshold voltage generating circuit. Referring to FIG. 1, a differential amplifier formed by npn transistors Q1 and Q2 is provided in a semiconductor integrated circuit 1, which is formed on a semiconductor substrate. The emitters of the transistors Q1 and Q2 are connected to one end of a constant current source 4 through resistors 2 and 3, respectively, while the other end of the constant current source 4 is grounded. The collector of the transistor Q1 is connected to a power source V.sub.CC, while the collector of the transistor Q2 is connected to the power source V.sub.CC through a resistor 5. A voltage obtained by dividing an external reference voltage 6 by external resistors 7 and 8 is applied to the base of the transistor Q1, while an internal reference voltage 9 is applied to the base of the transistor Q2. The external reference voltage 6 may be replaced by a voltage source 10 provided in the semiconductor integrated circuit 1, as shown by dotted lines in FIG. 1.
In operation, a current which is responsive to the base voltage difference between the transistors Q1 and Q2 flows to the resistor 5. A voltage drop V.sub.a is developed in the resistor 5 by this current, and is derived as a threshold voltage. The threshold voltage V.sub.a can be changed by adjusting the voltage dividing ratio between the external resistors 7 and 8.
FIG. 2 is a circuit diagram showing another conventional threshold voltage generating circuit. Referring to FIG. 2, a semiconductor integrated circuit 1 is provided therein with a voltage-to-current conversion circuit, which is formed by an operational amplifier 11, an npn transistor Q3 and a resistor 12. The output of the operational amplifier 11 is oonnected to the base of the transistor Q3. The emitter of the transistor Q3 is connected to a negative input of the operational amplifier 11, while being grounded through the resistor 12. The collector of the transistor Q3 is connected to a voltage source 14 through a resistor 13. A voltage obtained by dividing an external reference voltage 6 by external resistors 7 and 8 is applied to a positive input of the operational amplifier 11.
In operation, a current which is responsive to the voltage applied to the positive input of the operational amplifier 11 flows to the transistor Q3. This current also flows to the resistor 13, so that a voltage drop V.sub.a developed in the resistor 13 is used as a threshold voltage. Similarly to the circuit shown in FIG. a desired threshold voltage V.sub.a can be obtained by adjusting the voltage dividing ratio between the external resistors 7 and 8.
The conventional threshold voltage generating circuits have the aforementioned structures, each adapted to generate a voltage which is responsive to the reference voltage supplied from the exterior of the semiconductor integrated circuit through the amplifier provided in the semiconductor integrated circuit, to use this voltage as a threshold voltage within the semiconductor integrated circuit. Thus, the circuit is complicated in structure, and dispersion of threshold voltages is increased due to manufacturing dispersion of such integrated circuits. Because of a large number of components. The circuit is further complicated when a plurality of threshold voltages are generated.